International Patent Application WO91/12251 describes phthalazinones having bronchodilating and thromboxane A2 synthetase inhibiting properties. In the Intemational Patent Application W094/12461 and in the European Patent Application EP 0 763 534 3-aryl-pyridazin-6-one respectively arylalkyl-diazinone derivatives are described as selective PDE4 inhibitors.